Field
The present disclosure relates to techniques for transmitting and receiving optical signals. More specifically, the present disclosure relates to a polarization-insensitive optical transceiver.
Related Art
In order to provide a large bandwidth density, many optical communication systems use wavelength-division multiplexing (WDM). Moreover, in order to perform WDM it is often important to effectively perform wavelength filtering. Typically, WDM filters need to have: low loss, low crosstalk, flat passband, and accurate wavelength channel alignment with little or no tuning.
Silicon photonics is gaining increasing acceptance as the platform for photonic integration of optical communications. For example, submicron silicon-on-insulator (SOI) technology (with silicon-layer thicknesses typically ranging from 200 to 500 nm) can provide a very compact platform that enables optical propagation using highly confined optical modes and allows scaling integrated photonic devices down to the submicron level. However, WDM filters on silicon, such as ring-resonator based wavelength filters, are usually very sensitive to ambient temperature fluctuations because of the high thermo-optic (TO) coefficient of silicon (approximately 1.86·10−4/K). Because the temperature variation in a typical application environment can be 10s of degrees, active tuning is usually needed for silicon-based WDM filters.
However, active-temperature compensation techniques usually have high power consumption and can be difficult to implement, because they often use silicon/metal heaters, thermoelectric coolers (TECs) and closed-loop feedback controllers to maintain the local temperature. The two promising techniques for achieving passive athermalization of silicon are the use of materials with negative TO coefficients in optical waveguide claddings, and embedding a micro-ring in a thermally balanced interferometer.
The concept of using a negative TO coefficient in optical waveguide cladding is to balance the positive TO coefficients of the silicon core and silicon-dioxide substrate by engineering the optical mode confinement and the negative TO coefficient of the optical waveguide cladding. In practice, it can be difficult to implement this concept. For example, with polymer-based cladding materials, in addition to needing precise control of the material composition, it can be challenging to fabricate these materials in a manner that is compatible with CMOS processes. In particular, polymer materials often suffer from moisture absorption, chemical instability, UV aging, and poor mechanical characteristics.
Alternatively, titanium dioxide has a negative TO coefficient and is CMOS-compatible. However, in order to achieve a zero net thermal-optic coefficient of the optical waveguides, it is often necessary to re-engineer the optical mode such that it is less confined in the optical waveguide and is more distributed in the over-cladding region (e.g., by thinning or narrowing the optical waveguide, or by using a slotted structure). These techniques for reducing the optical mode confinement in the core typically result in an increase in propagation loss and bending loss, which further negatively impacts the resonator Q factor and footprint.
Instead of using a material with a negative TO coefficient, in another approach a ring resonator is optically coupled to a Mach-Zehnder interferometer (MZI). In this approach, the thermal drift of the ring resonator may be passively compensated for by tailoring the optical mode confinement in the optical waveguides in the MZI. While the use of the MZI can eliminate the need for new layers or materials, it can be difficult to fabricate the MZI because of strict constraints on the dimensions of the optical waveguides.
In addition to temperature dependence, another challenge for a submicron SOI platform is achieving polarization-transparent operation at the receiver. In particular, because of its high index-of-refraction contrast, submicron SOI circuits with a slab normally support the propagation of only one polarization mode (e.g., the TE-polarized mode). While, in principle, the TM-polarized mode is supported for very narrow optical waveguides (in a symmetric environment), in practice the effective index of refraction of such optical waveguides are very close to the index of refraction of the buried-oxide layer, so guiding is usually very weak (and, thus, there is usually high propagation loss). Furthermore, the polarization of the input light from an optical fiber is usually not fixed. Instead, the polarization state often changes because of deviations, such as elliptical cores, twists or bends, anisotropic stresses, temperature and pressure changes. Consequently, polarization-insensitive photonic devices and circuits are typically needed (particularly at the receiver) in order to capture the incoming light and to avoid performance degradation.
A variety of approaches have been considered for addressing the polarization-dependence of photonic devices and circuits, including: polarization-maintaining (PM) optical fibers, a polarization-state controller, and polarization-diversity systems (such as polarization splitters, rotators, and/or switches). A PM optical fiber usually requires intricate alignment at every splice and connector, and it is often costly and impractical to replace existing optical fibers. Moreover, a polarization-diversity technique typically involves dividing the input optical signal into two orthogonal components. These two orthogonal components are then routed separately to WDM filters, and recombined either optically or electrically to form the final output signal. However, the optical paths and circuits associated with the two polarizations generally need to be identical, and very accurate polarization states are usually required after rotation, which results in very stringent fabrication tolerances in order to implement these devices. Furthermore, polarization splitters and rotators in 501 often require high aspect-ratio features, extra layers, and/or an air cladding, which are usually not compatible with CMOS processes.
Hence, what is needed is an optical transceiver without the above-described problems.